1. Field of the Invention
The present invention relates to protection circuits, and in particular to an open protection circuit for a backlight module that uses fluorescent lamps.
2. General Background
Liquid crystal displays (LCDs) generally have the advantages of lightness in weight, a thin profile, flexible sizing, and low power consumption. For these reasons, LCDs are widely used in products such as laptops, personal digital assistants, mobile phones, and so on.
An LCD typically includes an LCD panel and a backlight module. The backlight module commonly includes fluorescent lamps, and acts as a light source for the LCD panel. Thereby, liquid crystal in the LCD panel can provide images for a display screen of the LCD panel. Conventional backlight modules for LCDs generally utilize pulse width modulators (PWMs) to control working currents of the fluorescent lamps. An open protection circuit is also provided to protect the PWM from damage when one of the fluorescent lamps fails or when an open circuit occurs for some other reason.
FIG. 3 is a diagram of a conventional open protection circuit of a backlight module that uses four fluorescent lamps. The fluorescent lamps are typically cold cathode fluorescent lamps (CCFLs). The open protection circuit 100 includes four detecting circuits 110, an input circuit 130, a PWM 150, and a switch 170.
Each of the detecting circuits has an output port 112, and is connected to a respective one of the fluorescent lamps 111. The input circuit 130 has four branches which correspond to the four fluorescent lamps 111. Each branch input circuit includes a bias resistor 132, a filtering capacitor 133, and a transistor. The transistors of the four branch circuits are designated 1331, 1332, 1333, 1334. The source terminal of the first transistor 1331 is connected to the drain terminal of the second transistor 1332. The source terminal of the second transistor 1332 is connected to the drain terminal of the third transistor 1333. The source terminal of the third transistor 1333 is connected to the drain terminal of the fourth transistor 1334. The source terminal of the fourth transistor 1334 is connected to ground. The drain terminal of the first transistor 1331 is utilized an output port of the input circuit 130.
The gate terminal of the first transistor 1331 in the first branch of the input circuit 130 is connected to ground via a bias resistor 132 and a filtering capacitor 135, which are arranged in parallel. The positive terminal of the diode 131 in the first branch acts as one of the input ports of the input circuit 130. The second, third, and fourth transistors 1332˜1334 in the other three branches are respectively connected to the other three bias resistors 132, filtering capacitors 135 and diodes 131 in similar arrangements to that of first branch. The positive terminals of the diodes 131 in the other three branches act as the other three input ports of the input circuit 130, in similar fashion to the diode 131 in the first branch.
The PWM 150 has a control port 151. The PWM 150 can be turned off or disabled when a low voltage level is provided to the control port 151 thereof.
The switch 170 includes a fifth transistor 171 and a current restriction resistor 172. The source terminal of the fifth transistor 171 is connected to ground, and the drain terminal of the fifth transistor 171 is connected to the control port 151 of the PWM 150. The gate terminal of the fifth transistor 170 and the drain terminal of the first transistor 1331 are both connected to a 5V power source provided by a pin of the PWM 150 via the current restriction resistor 172.
When the fluorescent lamps 111 are ignited and function normally, the four output ports 112 of the detecting circuits 110 are in a high voltage state. Thereby, the four output ports 112 enable the first through fourth transistors 1331˜1334, and pull the gate terminal of the fifth transistor 171 into a low voltage state. Simultaneously, the fifth transistor 171 is disabled. The control port 151 of the PWM 150 maintains a high voltage level, which keeps the PWM 150 functioning.
When any one of the fluorescent lamps 111 has an open circuit, the output port 112 of the corresponding detecting circuit 110 is pulled to a low voltage state, which disables a corresponding one of the four transistors 1331˜1334. The gate terminal of the fifth transistor 171 is then enabled by a high voltage provided by the 5V power source from the PWM 150. Simultaneously, the control port 151 of the PWM 150 is pulled to a low voltage state, and the PWM 150 is disabled.
At least one transistor 1331˜1334 and many passive electronic components are required for each one of the fluorescent lamps 111 in order to implement the open protection circuit 100. That is, the open protection circuit 100 is complicated and costly. This problem is even more pronounced in the case of a backlight module that has more than four fluorescent lamps 111. Hence, there is a need for a simpler and inexpensive open protection circuit for a backlight module.